VISION. 



533 



tlie choroid and ciliary processes. When the ciliary muscle, 

 which operates from a fixed point the corneo-selerotic junction, 



Fig. 388.— Illustrates mechaniem of accommodation (after Fick). The left side de- 

 picts the relation of parts during the passive condition of the eye (negative accom- 

 modation, or accommodation for long distances); the right side, that for near ob- 

 jects. 



pulls upon the choroid, etc., it relaxes the suspensory ligament; 

 hence the lens, not being pressed upon in front as it is from 

 behind by the vitreous humor (invested by its hyaloid mem- 

 brane), is free to bulge and so increase its refractive power. 

 The nearer an object approaches the eye, the greater the diver- 

 gence of the rays of light proceeding from it, and hence the 

 necessity for greater focusing power in the lens. 



If an animal be observed closely when looking from a remote 

 to a near object, it may be noticed that the eyes turn inward — 

 i.e., the visual axes converge and the pupils contract. These 

 are not, however, essential in the sense in which the changes 

 in the lens are ; for, as before stated, in the absence of the lens 

 distinct vision is quite impossible. 



ALTERATIONS IN THE SIZE OP THE PUPIL. 



The pupil varies in size according as the iris is in a greater 

 or less degree active. All observers are agreed that the circu- 

 lar fibers around the pupillary margin are muscular, forming 

 the so-called sphincter of the iris ; but great differences of opin- 

 ion still exist in regard to the radiating fibers. It is thought 

 by many that all the changes in the iris may be explained by 

 the elasticity of its structure without assuming the existence 

 of muscular fibers other than those of the sphincter ; thus a 

 contraction of the latter would result in diminution of the pu- 



